1. Technical Field
This invention relates to a functional film. The term “functional film” used herein is defined as follows. That is, the functional film is a film having a function, while the function denotes an action made through a physical and/or chemical phenomenon. The functional films include films having a variety of functions such as, for example, conductive film, magnetic film, ferromagnetic film, dielectric film, ferroelectric film, electrochromic film, electroluminescent film, insulating film, light-absorbing film, selective light-absorbing film, reflective film, anti-reflection film, catalyst film and photocatalyst film.
More particularly, the invention relates to a transparent conductive film. The transparent conductive film can be used not only as electroluminescent panel electrodes, electrochromic device electrodes, liquid crystal electrodes, transparent film heaters and touch panel transparent electrodes, but also as transparent electromagnetic wave shielding films.
2. Background Art
In the prior art, functional films formed of various functional materials are prepared by physical vapor phase deposition (PVD) techniques such as vacuum evaporation, laser ablation, sputtering, and ion plating and chemical vapor phase deposition (CVD) techniques such as thermal CVD, photo CVD and plasma CVD. These techniques generally need large size apparatus, some of which are inadequate to form large surface area films.
Also known in the art is the formation of film by coating using a sol-gel process. The sol-gel process is suited to form large surface area films, but often requires sintering of inorganic material at high temperature after coating.
Referring to transparent conductive films, for example, their detail is described below. At present, transparent conductive films are prepared most often by a sputtering process. The sputtering process, though including a variety of ways, typically involves generating inert gas ions in vacuum by a DC or high-frequency discharge, accelerating the ions for impingement against a target surface, ejecting target constituent atoms from the surface, and depositing them on a substrate surface to form a film.
The sputtering process is advantageous in that a conductive film with a low surface resistivity can be formed even over a relatively large surface area. However, it has the drawbacks of a large size of apparatus and a slow deposition rate. If the surface area of conductive film will increase in the future, the apparatus will be further increased in size. This means that technical problems arise in that a higher precision of control is needed, and from an economical aspect, the manufacturing cost increases. As a complement to the slow deposition rate, it is customary for the sputtering process to use an increased number of targets to increase the deposition rate, which also causes the apparatus to increase in size.
It has also been attempted to form transparent conductive films by a coating process. The conventional coating process forms a conductive film by applying a conductive paint having conductive microparticulates dispersed in a binder solution onto a substrate, followed by drying and curing. The coating process has advantages including ease of formation of conductive film having a large surface area, use of simple apparatus, high productivity, and preparation of conductive film at a lower cost than the sputtering process. In the coating process, conductive microparticulates contact with each other to form conductive paths exerting electric conductivity. However, conductive microparticulates are in insufficient contact in the conductive film prepared by the conventional coating process, resulting in the drawback that the conductive film has a high resistivity or poor electrical conductivity and its use is restricted.
As a typical method of preparing transparent conductive films by the conventional coating process, for example, JP-A 9-109259 discloses a preparation method involving the first step of applying a paint of conductive powder in a binder resin to a transfer plastic film and drying the coating to form a conductive layer, the second step of treating the conductive layer under a pressure (5 to 100 kg/cm2) and heat (70 to 180° C.) for smoothing its surface, and the third step of stacking such conductive layers on a plastic film or sheet followed by thermocompression.
This method fails to produce a transparent conductive film having a low resistivity because a large amount of the binder resin is used, specifically 100 to 500 parts by weight of conductive powder per 100 parts by weight of the binder in the event of inorganic conductive powder, and 0.1 to 30 parts by weight of conductive powder per 100 parts by weight of the binder in the event of organic conductive powder.
Also, JP-A 8-199096 discloses a method applying to a glass plate a conductive film-forming paint consisting of tin-doped indium oxide (ITO) powder, a solvent, a coupling agent and an organic or inorganic acid salt of metal, but free of a binder and firing at a temperature of 300° C. or higher. Since no binder is used in this method, the conductive film has a low resistivity. However, since the firing step at a temperature of 300° C. or higher is essential, it becomes difficult to form a conductive film on a resin film or similar support having a low heat resistant temperature. Specifically, the resin film can be melted or carbonized or burned when heated at high temperature. The heat resistant temperature of resin film varies with the identity of resin, and polyethylene terephthalate (PET) film, for example, presumably has a limit temperature of 130° C.
Aside from the coating process, JP-A 6-13785 discloses a conductive coating including a consolidated powder layer in which at least some, preferably all, of voids in a skeletal structure made of conductive material (metal or alloy) are filled with resin, and an underlying resin layer. Its preparation method is described by referring to the formation of a coating on a plate as a typical example. According to the above-cited patent, a resin, a powder material (metal or alloy) and a plate serving as a member to be treated are first vibrated or agitated in a vessel along with a coat-forming medium (steel balls having a diameter of several millimeters) whereby a resin layer is formed on the surface of the member to be treated. Then, the powder material is captured and secured to the resin layer by the tack of the resin layer. Further, the coat-forming medium being subject to vibration or agitation applies impact forces to the powder material being subject to vibration or agitation whereby a consolidated powder layer is formed. This method requires a substantial amount of resin in order to exert the anchoring effect of the consolidated powder layer. Additionally, this method is more complicated than the coating process.
Yet aside from the coating process, JP-A 9-107195 discloses a method involving spraying and depositing conductive short fibers on a film of PVC or the like, followed by pressure treatment to form a conductive fiber/resin integrated layer. The conductive short fibers used herein include short fibers of polyethylene terephthalate or the like which have been metallized as by nickel plating. The pressure treatment is preferably effected under temperature conditions where the resin matrix layer exhibits a thermoplastic behavior, for example, high temperature/low pressure conditions of 175° C. and 20 kg/cm2 as described in the above patent.